The present invention relates to a resistance welding control system for securing a sound joint condition in the resistance welding process.
Generally, a study of the behavior of an interelectrode resistance R i.e. the resistance between two electrodes for securing a sound joint condition in the resistance welding process shows that the interelectrode resistance R assumes a waveform having a peak value (See FIG. 12) about when a nugget (molten part) begins to form at the joint after the temperature thereof reaches a level higher than the melting point of the base metal.
It is thus possible to secure a proper nugget area for producing a satisfactory joint condition at a peak value of the interelectrode resistance which is equal to a maximum resistance value Rmp less a resistance drop .DELTA.R.
In view of this, unlike in the conventional control systems in which the welding conditions including the welding current and energization time are fixed in open loop control, there have recently been suggested control systems aimed at securing a sound welding quality by stopping the energization at a resistance drop of .DELTA.R from a maximum resistance value Rmp in accordance with the waveform of the above-mentioned interelectrode resistance R. (See, for example, Japanese Patent Publication No. JP-A-57-109581).
In the aforementioned art of stopping energization with a resistance drop of .DELTA.R from the interelectrode resistance R, the temperature rise of the joint varies due to a change in the surface conditions of the base metal or the electrode conditions so that the time at which the resistance drop value .DELTA.R appears undesirably changes.
The result is a variation in energization time. If the value .DELTA.R is reached within a short time, for instance, a welding fault such as a welding spark would be generated, while if it takes a considerable time before .DELTA.R is reached, on the other hand, the recess of the weld zone grows to such an extent that the welding strength would be deteriorated, thus making it impossible to secure a satisfactory joint condition in steady fashion.